RE: [SI-LIST] : Simple question about propagation speed vs. dielectric

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From: Doug McKean (dmckean@gte.net)
Date: Sun Feb 18 2001 - 17:40:05 PST


Keith wrote:
>
> Folks,
> In general, is it true or false,
> signals travel faster the lower
> the Er or not, why or why not?

True. If you're talking about the signal
propagating through JUST the material which
has that Er as a characteristic, then using
V = C/sqrt(Er) is valid.

Conceptually think of it as the signal having
to build more or less of an electric field
depending upon the value of Er being higher or
lower respectively. Higher Er, more of an
electric field to be built, more time. There's
a lot of assumptions in there, but it's about
as simple as that.

Signals traveling just through air which has
an Er of about 1, would have a velocity of
about C. Signals traveling just through FR4
material would have a velocity at about 1/2
that of air.

Prop delay of a signal traveling down a
perfect transmission line with no load,
I estimate with

   Tpd = [L0 * C0]^0.5 [time/length]

L0 = inductance per unit length
C0 = capacitance per unit length

The Er of the dielectric material around the
trace is embedded in the C0 term. The higher
the value of Er, the longer the prop delay.

Regards, Doug McKean

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